also sent to the white house, to president obama. it also has tested positive for ricin and is awaiting confirming tests from the lab. both letters appear to be written by the same person, both postmarked april 8th, sent from memphis, tennessee, both said "to see a wrong and not expose it is to become a silent partner to its continue yuns." i am kc and i approve this message. part of his initials are, in fact, k.c. his name is paul kevin curtis, and he is from mississippi, just north of tupalo, mississippi. we do not know anything about him yet or the evidence against him other than the fact he has been arrested. in terms of what was in the letters, what substance was in the letters, federal officials tell nbc news tonight that initial lab tests of the letters, lab tests, not the field tests, the initial follow-up lab tests were inconclusive. there was some level of ricin measured in the lab tests, but the exact potency is as yet unclear, they say more tests are needed. joining us now to help us understand what this means is michael